If everyone smoked twenty cigarettes a day the difference between those who got lung cancer and those who didn’t would be almost 100% heritable, even though the cause would be almost 100% environmental. Heritability depends on our environment.

It is believed that IQ is around 70% heritable, if all children were to have an educationally rich environment in which to grow then, due to this environment, the effect of heritability on IQ would increase. If children were brought up in an educationally damaging environment the effects of heritability on IQ would reduce dramatically.